1. Field of the Invention
The present invention relates generally to electrical test equipment, and particularly to electrical test equipment for testing the integrity of electrical power distribution systems and for locating faults in the same.
2. Technical Background
Residential dwellings, office buildings and businesses in the United States are typically wired in accordance with the National Electrical Code, inspected after initial construction, and then never again tested unless a problem occurs. Yet, after 10, 20, or 30 years of continuous use, wire insulation dries out and cracks, contacts become loose, light sockets degrade from heat, switches lose their spring, and numerous other aging processes take their toll. Additions and modifications to the electrical wiring over the years, either by certified electricians or perhaps a handyman, or even the owner herself add another level of uncertainty about the wiring. The difficulty is that these changes over time may eventually lead to a fire hazard, a shock hazard, unexpected failures and costly repairs, or perhaps all of these things.
Degraded or loose contacts and overloaded circuits can lead to contact arcing, a process that starts intermittently but given enough time may become persistent. Electrical arcs even at low currents can develop temperatures that exceed the ignition temperature of most common flammable materials and therefore pose a significant fire hazard. Prudently, in response to the electrical problems of the past, the National Electrical Code has been developed to mitigate this danger. For example, the code specifies that all wiring connections be contained in non-flammable junction boxes thereby reducing the probability that arcing at a connection will come into contact with flammable materials. Nevertheless, connections sometimes degrade over time and begin arcing, causing flickering lights, circuits to go out, and sometimes a fire. The US National Fire Protection Association estimates there were 38,300 residential fires of electrical origin in 1998 resulting in 284 deaths and 1184 injuries and $668.8M in direct property damage (from NFPA's U.S. Home Product Report, Appliances and Equipment Involved in Fires, January 2002). According to the NFPA “Electrical distribution equipment (i.e., wiring, switches, outlets, cords and plugs, fuse and circuit breaker boxes, lighting fixtures and lamps) was the third leading cause of home fires and the second leading cause of fire deaths in the United States between 1994 and 1998”. But even when arcing does not result in a fire, it can cause electrical interference, flickering lights, intermittent service and eventually substantial damage to the arcing contacts themselves as well as flammable materials in the vicinity.
Aging wiring and/or incorrect repairs can also lead to wiring faults that present a shock hazard for building occupants. The loss of grounding protection, for example, particularly in wet areas, can present the danger of electrocution. The US Consumer Product Safety Commission estimates that of 230 electrocution deaths involving consumer products in the United States in 1995, 23% resulted from installed home wiring.
In addition to the danger implicit in developing electrical faults, the cost of waiting until faults develop into a noticeable problem can be high. Arcing on loose or dirty contacts of a circuit breaker, for example, will eventually lead to the need to replace the entire Load Center.
And finally, the electrical wiring in older buildings is often not well documented. The label on the Load Center of an older building is often barely legible or in error. The process of correcting the label, i.e., determining which circuit breakers control which circuits, can be quite time consuming, often requiring more time than is warranted absent a problem. Without adequate documentation, diagnosing problems is more difficult and branch circuits may be inadvertently overloaded.
A number of tools and instruments are commercially available to test and diagnose various aspects of household wiring. Inexpensive plug-in modules, for example, are commonly available to test whether a grounded socket is wired properly. Plug-in Ground-Loop Testers have also been marketed over the years to test the current-handling capacity of ground return paths. Common electrical test equipment, such as hand-held voltmeters and the like, and more sophisticated instruments such as oscilloscopes can also be used to test home electrical wiring; however, these tools yield only limited information. A thorough test of all the wiring in a home done manually would likely take a couple of electricians many days to complete and would therefore not be economically feasible.
There exists a need therefore for an instrument and method to quickly and economically test the entire electrical wiring system in a home, office, or business, so as to identify and locate dangerous conditions and flaws, and thereby reduce the chance of electrical fires and to protect occupants from the danger of electrical shock. A less important advantage of such testing would be to accurately document the system in the process.